To see whether the acute natriuretic effect of nifedipine is accompanied by changes in atrial natriuretic peptide levels, a group of eight hypertensive patients were studied. After at least a week of constant sodium intake, placebo or nifedipine (10 mg s.I.) were administered and blood pressure, heart rate, plasma renin activity, plasma aldosterone and atrial natriuretic peptide plasma levels, urinary sodium, urinary volume and creatinine clearance were monitored for 2 hours. While placebo did not induce changes in any of the above parameters, nifedipine administration was followed by a significant decrease in blood pressure and an increase in urinary sodium, urinary volume and creatinine clearance; these changes were accompanied by a significant rise in atrial natriuretic peptide levels from 19.4 +/- 2.8 pg/ml to a maximum of 23.9 +/- 2.5 pg/ml and 24.1 +/- 2.2 pg/ml (P less than 0.05) at 60 and 90 minutes, respectively. In conclusion, our data do not rule out the possibility that atrial natriuretic peptide participates in the nifedipine-induced increase in sodium and water excretion.